Washington — The most extensive genetic analysis of mountain gorillas ever conducted has found the critically endangered apes burdened with severe inbreeding and at risk of extinction but the researchers still see reasons for optimism about their survival.

Twenty-three scientists from six countries unveiled on Thursday the first complete genetic map of the mountain gorilla, a close genetic cousin to humans inhabiting two isolated areas in central Africa.

The study in the journal Sciencerevealed a substantial loss of genetic diversity from inbreeding caused by mating with close relatives due to small population size, with mountain gorillas inheriting identical segments from both parents in about a third of their genome.

Inbreeding can increase threats from disease and environmental change by reducing the genetic ability to adapt and cause a larger hardship of harmful mutations.

"Mountain gorillas are critically endangered and at risk of extinction, and our study reveals that as well as suffering a dramatic collapse in numbers during the last century, they had already experienced a long decline going back many thousands of years," University of Cambridge geneticist Aylwyn Scally said.

The researchers were surprised that many of the most harmful mutations, those that can stop genes from working and cause serious health conditions, were less common than in other gorilla subspecies.

"We have shown that although low in genetic diversity they have not yet crossed any genetic threshold of no return. They can continue to survive and will return to larger numbers if we help them," Scally said.

There are only about 880 mountain gorillas, living in mist-covered forests of the Virunga volcanic mountain range on the borders of Rwanda, Uganda and the Democratic Republic of Congo and Uganda's Bwindi Impenetrable National Park. The study was based on blood samples from seven Virunga gorillas.

Two gorilla species exist, eastern gorillas and western gorillas. Mountain gorillas, with thicker and longer fur than other gorillas, are one of two subspecies of eastern gorillas.

"While comparable levels of inbreeding contributed to the extinction of our relatives the Neanderthals, mountain gorillas may be more resilient," Copenhagen Zoo geneticist Christina Hvilsom said.

The researchers said the main threats to these gorillas are from humans: habitat loss, hunting and diseases transmitted from people. Tyler-Smith said, "We just need to continue to conserve them: their future lies in our hands."